Given the importance of battery technology in enabling of the profusion of portable/mobile electronic and cordless electric devices, battery materials with improved performance and lower cost are of continuing interest. Electric and hybrid vehicles are another major application for high performance battery technology. Commercially, the most important battery class under development today, one having the highest energy density and excellent cycling performance, is the lithium ion battery. Although performance improvement is an ongoing goal, the biggest challenge for lithium ion batteries is achieving low cost.
Research and development is active around the world in all aspects of lithium ion battery composition, design, and manufacturing. Of particularly high importance are efforts aimed at the development of new lower cost, high performance anode and cathode electrode materials, and low cost material preparation methods. The safety of lithium batteries is also a general concern.
A known class of cathode materials receiving attention is that of the lithium metal-silicates having the general formula Li2MSiO4, wherein M can be Fe, Mn, or Co. These materials crystallize as layered intercalation compounds in which Li ions can freely drift in and out of the structure. Of particular interest is the possibility that up to 2 Li ions per formula unit may be extracted from, and returned to, the structure in a given charge/discharge cycle, thereby leading to very high charge storage capacities. While this remains an attractive goal for continuing development of these materials and lithium ion battery cells that incorporate them, this theoretical advantage has not yet been realized in a practical cell.
Once more, cost is a major concern for commercial applications, and from the point of view of raw element cost, and potentially high storage density, the Li2MSiO4 materials offer the opportunity for an improved cost/performance ratio in lithium ion battery technology. Of course material processing cost must also be considered, as material synthesis/processing technology is an important contribution to the overall cost. In general, traditional solid state synthetic methods use substantial amounts of time and energy to achieve a level of comminution of the input materials required to form sufficiently small particles that eventually enable the solid state reaction of the input materials to progress during a calcining and/or a sintering step that gives rise to the desired reaction product phase. This high temperature reaction portion of the process, known as calcining, is energy intensive, requiring both high temperatures and long times a high temperature, because the process relies on diffusion in the solid state over relatively long distances, given the particle-sizes typically achieved by mechanical milling of input materials.
Materials in the Li2MSiO4 family have been produced by a number of routes including traditional solid state ceramic processes. Aravindan et al. in J. Mater. Chem., 2011, 21, 2470, entitled “Influence of carbon towards improved lithium storage properties of Li2MnSiO4 cathodes” disclose a process that uses a stoichiometric solid state synthesis, wherein the starting materials LiOH hydrate, MnCO3, and SiO2 are combined in a stoichiometric manner. They report that the Use of tetraethylene glycol (TEG) in a solvothermal process results in the formation of tarry thermal decomposition by-products that are tedious to remove, and are difficult to control. A sintering temperature of 900° C. is reported, whereby the resulting Li2MSiO4 material is not phase-pure, and scanning electron microscopy (SEM) reveals grain sizes in the 1 to 10 micrometer range with substantial particle aggregation. The use of adipic acid as a gelling and capping agent to assist in the synthesis and to prevent aggregation during sintering is also Reported.
US 2011/0269022 A1 to Takahiro Kawakami and Masaki Yamakaji discloses the synthesis of Li2MnSiO4 and Li2FeSiO4 by a traditional solid state synthetic approach, the latter from lithium carbonate, iron oxalate and silicon dioxide. Also disclosed are positive electrode materials comprised of Li2MSiO4, wherein M can be Fe, Mn, or Co, alone or in combination. Sintering temperatures reported are in the range of 700-100° C.
In “Microwave-Solvothermal Synthesis of Nanostructured Li2MSiO4/C (M=Mn and Fe) Cathodes for Lithium-Ion Batteries” by T. Muraliganth et al., Chem. Mater. 2010, 22, 5754-5761, the solvothermal preparation of the title compounds is disclosed. Tetraethylene glycol (TEG) is reported as the solvent for the process. Lithium hydroxide, manganese acetate, and tetraethyl orthosilicate (TEOS), in stoichiometric ratios of the cations, are combined in TEG solvent and sealed in a closed reaction vessel. The mixture is heated under stirring by microwave excitation to a temperature of 300° C. and a pressure of 30 atmospheres. However, the requirement of high-pressure sealed-reactor processing is a significant detriment to low cost, high volume manufacturing of Li2MSiO4. The reaction product was washed repeatedly in acetone to remove unwanted reaction by-products, and the authors report a high air sensitivity of the reaction product. The authors report single phase target product after a process step of heating at 650° C. (in Argon) of the solvothermal reaction product, the single phase target product is characterized by an average grain size of about 20 nm. The small grain size results from the solution-based preparation and is beneficial because of improved ionic and electronic conduction. Because of the low intrinsic electronic conductivity of the Li2MSiO4 materials, it is common in the art to coat the Li2MSiO4 particles with carbon. The authors report poor rate performance and severe capacity fade during cycling of the Li2MSiO4/C composite.
What is needed is a process for forming cathode materials of the class Li2MSiO4 having a low cost preparation process that produces a lithium ion battery with superior charge storage density, good rate performance, and extended cyclability, while offering safety in end-use applications.